10. Summary: The E.-W. dikes of quartz-dolerite and tholeiite in Scotland and the related sills show marked resemblances, both chemical and mineralogical, to the Whin Sill in the north of England. They appear to belong to the same period of intrusion, which is roughly coeval with the Permo-Carboniferous folding and faulting. Chemical analyses of an unaltered residual glass in one of the tholeiites shows it to be highly siliceous, and there is therefore no reason for considering the micropegmatite of the very similar quartz-dolerites to be other than magmatic. It is suggested that the suite may have arisen through mild contamination of the parental Carboniferous olivine-basalt magma by prolonged contact with the ‘sial’.